Because of the nature of the human eye and its characteristics of free movement in an eye socket, it is imperative that the eye be restrained during the cutting or shaping of the cornea thereof. In U.S. Pat. No. 5,556,406, issued Sep. 17, 1996, and owned by the assignee of the present application, there is described a technique for holding the anterior cornea surface with a template during the cutting thereof with a waterjet surgical cutting device, in a refractive vision correction procedure. This helps to stabilize the cornea during the cut and, in fact, it seems to be essential to get a smooth cut. The template works particularly well when it is provided with a layer of sharp microscopic size particles or other similar structural elements on the surface of the template which bite into the cornea surface as the cornea is pressed against the template. This further helps in preventing lateral motion of the cornea surface during the cutting. Movement stability during cutting, even with a waterjet, is essential since dimensional tolerances of microns are required in order to achieve accurate corneal refraction correction.
Surgical knife blade-based (microkeratomes) are unable to achieve such accuracies even with effective corneal restraint. With use of a surgical knife blade to cut through the cornea, corneal tissue slips and slides past the blade, the tissue is compressed and distorted, and the cut layers are wedged, rippled, striated, etc. A good analogy to the action of this blade based microkeratome is the operation and inadequacies of a carpenter's plane. In addition, epithelial material, dragged in by the blade, may be left on the interface between the cut layers, which, after the layer is replaced, may grow later within the stroma and cause significant difficulties. Tiny chips of the blade edge may also be left imbedded in the stroma. None of this occurs with a waterjet cut. It should also be noted that a surgical blade has a wide portion of at least 150 microns of the blade part, as distinct from the edge, and this must displace tissue as it cuts. Thus, the forces on the tissue are larger.
A waterjet of a waterjet tissue cutting system may be made to operate readily at pressures as high as 20,000 psi, with a scan of the waterjet beam across the cornea at speeds well above 10 mm per second. At such speeds the waterjet moves a distance equal to its own diameter in a few milliseconds, faster than the cornea can respond mechanically to the forces exerted by the waterjet. Thus, under such conditions the cornea behaves almost like a rigid body, but without the drawbacks of an actual rigid body, an ideal situation for precise cutting. However, with normal lateral cuts effected through the cornea, a waterjet still encounters layers (epithelium, Bowman's layer, stroma) of varying density and toughness which may detrimentally affect cutting control, accuracy and integrity of cuts. In addition, the lateral action of the waterjet is similar to a cut with a drawn blade. The waterjet laterally "cuts" from its leading edge by effecting a separation between lamellae of corneal tissue during scanning rather than by actually cutting through the lamellae which is a lower energy procedure. If the waterjet is also required to effect the initial cross lamellae cut excessive energy is required for the waterjet during the cutting procedure.
With a prior art lateral waterjet cut through the corneal tissue there is an initial incision through the curved outer surface of the cornea. Thus, the beginning and end of the cut are oblique cuts through the tough outer layer surface of the cornea (epithelium and Bowman's layer) before a cut is effected in the stroma tissue as desired. A gutter formed with the lateral cut is not perfectly defined and there is some residual structure at the boundary transition.
The keratomileusis (lamellar keratoplasty) procedure is one involving lateral cuts of corneal tissue which initially involves surgical removal, with a microkeratome, of a uniform thickness button or lenticule of corneal tissue of a thickness containing the epithelium layer, Bowman's membrane (intact) and a portion of the stroma. The button or lenticule preferably remains hingedly attached at one point to the cornea as a replaceable flap. The lenticule is moved out of the way, the stroma bed is then surgically reshaped, with a knife blade or laser, as required, and the lenticule is replaced. However, the lack of a perfect fit of the flap on the stromal bed, when the scalpel type of microkeratome is used, is a source of irregular astigmatism.